The present invention relates to a retaining ring assembly for use on a carrier head in a chemical mechanical polishing apparatus. Chemical mechanical polishers are used in several applications including the manufacture of integrated circuits where they provide the silicon wafer substrates with a smooth flat finish prior to and during the sequential deposition of conductive, semi-conductive and/or insulative layers thereon. The polishing is accomplished by placing the semi-conductor wafer on a carrier head of the polisher which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer and a retaining lip or ring around the edge of the wafer to keep the wafer centered on the carrier head. The front side of the wafer is then contacted by a rotating polishing pad in a chemically reactive slurry that polishes the outermost surface of the wafer to a flat smooth surface. During the polishing, the carrier head and retaining ring assembly maintain a bias force between the surface of the substrate and the rotating polishing pad. The movement of the slurry-wetted polishing pad across the surface of the substrate causes material to be mechanically and chemically polished (removed) from the face of the substrate.
A common problem with chemical mechanical polishing is the rapid wear on the parts as the substrates are polished. The retaining ring assembly mounted under the substrate carrier continually wears as the polishing pad makes direct contact against the featured substrate layer surfaces. Because the retaining ring assembly must be substantially parallel to the polishing pad during the polishing process to avoid surface irregularities, use of a worn ring assembly would create a defective product. Accordingly, frequent replacement of the retaining ring assembly on mechanical chemical polishers is a necessary and frequent occurrence.
The retaining ring assemblies on chemical mechanical polishers were previously constructed entirely of a plastic material. To add rigidity to the assembly, two-piece retaining ring assemblies were developed that comprised a lower or forward retaining ring portion formed of a plastic material that was permanently affixed to a stainless steel backing. The entire assembly could not be formed of stainless steel as the stainless steel would then contaminate the lapping. As a result, when the lower plastic retaining ring portion of the assembly became worn, the entire assembly still had to be discarded and replaced. Because of the high costs of these assemblies, frequent replacement represents a significant expense.
In an effort to reduce the costs of replacing these retaining ring assemblies, a detachable two-piece ring assembly was developed. Such an assembly employed a reusable annular ring backing formed of stainless steel and a resilient plastic retaining ring portion releasably secured to the backing by a dove-tailed snap fitment. Such a ring configuration is disclosed in U.S. Pat. No. 6,186,880 and enables the metal backing to be reused when the plastic retaining portion becomes overly worn and requires replacement. While such ring assemblies may reduce the cost of part replacement, the press fitment provided by the mating relationship between the retaining and backing rings requires tight tolerances. It also relies on the plastic material of which the retaining ring is constructed to effect the securement. Such material is notch sensitive so that any cracks in the material may cause the retaining ring to break upon being compressed as it is forced into the smaller channel in the metal backing. The material is also heat and chemically sensitive, which also can lead to destruction or damage of the retaining ring under the stresses of the snap fit attachment.
It would be highly desirable to provide a two-piece retaining ring assembly for use in chemical mechanical polishers which reduced the cost of part replacement by providing a releasable securement between the metal backing and plastic retaining ring portion but which did not require such tight tolerances in the ring, nor relied on the deformation of the plastic material to effect securement. The result would be a significant reduction in costs of part fabrication and in breakage problems resulting from material imperfections during assembly. The ring assembly of the present invention obtains these results.